In the mounting of an integrated circuit package to a printed board (PCB), or similar support substrate, there are different methods of providing electrical interconnections between the package's input/output (I/O) pads and the substrate's pads. One method of interconnection, referred to as land grid array interconnection, uses a land grid array socket. This socket comprises a plurality of compressible conductive elements placed between the IC package and the substrate.
The compressible conductive elements are maintained in an array that corresponds to the package I/O pad array, by means of an electrically insulating matrix that acts as a contact support, or retainer. The land grid array socket is placed between the package and substrate. The IC package and substrate are held together with appropriate compression hardware, thereby causing each and every conductive element to be compressed and thus create appropriate contact force between its corresponding IC package pad and substrate pad, which ensures an electrically conductive path between those pads.
An example of a land grid array socket is shown in U.S. Pat. No. 4,922,376 to Pommer et al, entitled "Spring Grid Array Interconnection for Active Microelectronic Elements" and issued May 1, 1990. This patent describes the use of a plurality of conductive resilient elements, held in an insulating contact retainer having a plurality of apertures to hold each conductive element. The resilient elements provide an electrical connection between pads of the IC package and those of a substrate.
U.S. Pat. No. 5,139,427 to Boyd et al., entitled "Planar Array Connector and Flexible Contact Therefor", issued Aug. 18, 1992, shows another type of socket that can be used for land grid array interconnections. The individual contacts are formed of flat metal stock having spring characteristics including spring elements, held in a planar array by means of an insulating sheet fitted with openings, each opening containing a conductive spring contact.
In U.S. Pat. No. 5,273,438 to Bradley et al., entitled "Canted Coil Spring Array and Method for Producing the Same", issued Dec. 28, 1993, electrical contact in the land grid array socket is provided by an array of canted coil springs.
U.S. Pat. No. 5,033,970 to Buchoff, entitled "Self-mounted Chip Carrier" and issued Jul. 23, 1991, shows an elastomeric ring that provides electrical conductive paths, instead of individual elements as in the other documents cited above.
U.S. Pat. No. 5,832,601 to Eldridge et al., entitled "Method of Making Temporary Connections Between Electronic Components", issued Nov. 10, 1998, describes a method of attaching wires onto semiconductor dies, and use of these wires as resilient contact structures to perform electrical testing of the dies. This patent describes a method of manufacturing resilient contact structures, with soft core wires attached to a tip made from a sacrificial substrate, and subsequently coated with a hard material.
IC packages used for land grid array interconnection are usually ceramic or plastic type packages and, according to the present state of the art, as described above, have no interconnection means permanently attached to the metallized conductive I/O pads thereof.
Another commonly used method for electrically connecting integrated circuit packages to substrates is known as surface mounting. For this type of connection, leads, solder balls, or any other solderable interconnection means are permanently attached to the IC package; which in turn are permanently attached to the printed circuit board pads by means of soldering. U.S. Pat. No. 4,751,199 to Phy, entitled "Process for Forming a Complaint Lead Frame for Array-type Semiconductor Packages", issued Jun. 14, 1998, shows one example of a surface mountable component, with discrete complaint conductive elements suitable for surface attachment to the printed circuit board by conventional surface mount assembly soldering.
Comparing the aforementioned methods, it is understood that land grid array interconnections consist typically of a non-permanent mechanical assembly between the package and substrate, the package being demountable. This important advantage (demountability) is not available in the case of known surface mount assemblies. Furthermore, land grid array offers a more complaint interconnection than surface mount assembly, thereby offering improved reliability when such packages are submitted to thermal and mechanical stresses. On the other hand, land grid array interconnection methods typically require the use of a land grid array socket, which adds significant costs to the assembly. Use of a socket also requires precise alignment between the socket conductive elements and the package I/O pads, in order to provide the required conductive path to the substrate pads. Surface mount interconnection requires no such socket, since surface mount leads are already attached to the package before it is assembled to the printed circuit board.
Some work has been done by the assignee of the subject application in order to provide an electrical circuit package that could offer the combined advantages of surface mount and land grid array interconnection methods. For example, International Business Machines (IBM) Technical Disclosure Bulletin, September 1994, pages 395-396 entitled "High Density Complaint Connector and socket", teaches the use of wire balls attached to an IC package, in order to provide a high density demountable interconnection.